Coal-fired power plants equipped with the latest thermal process to extract carbon dioxide (CO2) burn 30% more coal than plants that do not extract CO2. CO2 capture requires more coal because it creates a 24% parasitic energy demand by CO2 absorption, desorption, and compression unit operations. Because of this huge added cost, operators of coal-fired plants are resistant to adding CO2 capture technology.
Several international treaties on the Earth's climate at Montreal, Kyoto, and Copenhagen have demonstrated the importance of global regulation of emissions of greenhouse gases, particularly CO2. According to the EPA, the process of generating electricity is the single largest source of CO2 emissions in the United States, representing 41% of all CO2 emissions, with over 80% of the CO2 from electricity generation coming from coal-fired power plants. Unfortunately, coal will continue to be used as a major source of power generation in the U.S. for the foreseeable future, and according to the World Coal Institute, coal's share in global electricity generation is set to increase from 41% to 44% by 2030. With this in mind, reducing the environmental impacts of coal, especially CO2 production, is vital, and capturing CO2 in the most efficient and cost effective manner is critical for the industry.
Two major reviews in 2009 discussed three CO2 capture concepts for coal-fired power plants—(a) post-combustion capture, (b) oxy-combustion capture, and (c) pre-combustion capture; with the post-combustion capture technology being the most efficient, cost effective, and most adopted today. There are four major categories of current technologies for post-combustion CO2 capture. These are (i) amine absorption; (ii) reactive oxide/carbonate solids; (iii) zeolite absorption, and (iv) membranes. The challenges for adoption of each of these technologies have been discussed by Rochelle et al. in 2009. Amine absorption of CO2 is the most advanced, most well understood, and most successful method, with monoethanolamine (MEA) as the most widely deployed amine in CO2 capture in the industry. The United States Department of Energy also considers the amine solvent process as the current state-of-the-art in capture technology. Unfortunately, even using the MEA technology dramatically drops a power plant's overall thermal efficiency from 39% to 29%. In an amine solvent process, CO2 is readily absorbed by an amine solution from flue gas. However, thermally extracting the CO2 from the amine solution is energy intensive and incurs burning 30% more coal than is necessary to generate electricity. The thermal process increases the cost of electricity (COE) by 81% for a supercritical pulverized coal plant. The bulk of this parasitic energy is used for maintaining steam boilers that provide 100-120° C. temperature for thermal desorption of CO2 from an amine scrubber.
Recently, using Dow Chemical Company's 30% MEA for scrubber/separation processes of CO2 capture has produced better efficiencies than with 20% MEA of Kerr-McGee, by reducing the amount of energy expended from 0.51 to 0.37 megawatt-hours (MWh) per metric ton of CO2 removed. The decrease in required energy has reduced the cost for the removal of CO2 from $82 to $51 per ton (t), but has still left us with an increase in the COE of 81%. The photochemical CO2 separation technology in this proposal has the potential to reduce the COE to 35%. This is significant for reaching the goal of less than 35% COE. A review by Ramezan et al. has concluded that this incremental improvement to the thermal process has yet to come close to achieving the DOE goal (and potentially EPA's regulatory standard) of 90% CO2 removal with less than a 35% increase in COE. The disclosed photolysis process has the potential to meet this goal.